Solid phase microextraction (SPME) is a demonstrated and attractive method for sampling and preconcentrating trace level analytes, since it is a flexible, rapid, solvent-free extraction technique that is applicable to liquid and/or gas sampling environments. Further, SPME can be directly interfaced with wide range of analytical instrumentation for analyte detection such as gas chromatography (GC), gas chromatography mass spectroscopy (GC-MS), high-performance liquid chromatography (HPLC), liquid cromotography mass spectroscopy (LC-MS), and desorption electrospray ionization mass spectroscopy (DESI-MS), making it amenable to automation and a convenient technique. Due to its broad applicability and sensitivity SPME has been widely utilized in a variety of fields.
SPME was first successfully developed as a polymer coating on silica fibers in the early 1990s and, presently, various polymer phases are available commercially. Many types of polymers and composites with different chemical characteristics are available for the extraction of a variety of analytes of interest. Since polymer phases can be blended, different surface properties of polymers can simply be coated and tailored onto a single fiber. This leads to a certain extend application of polymer coated fibers in many fields.
However, a number of drawbacks of polymer SPME sometimes limit their applications. They are unstable with some solvents, have insufficient mechanical strength and tend to degrade at high operating temperatures. Additionally, the coatings are occasionally stripped, and they suffer from contaminations and unstable coatings on new fibers.
The demand for better chemical capacity, sensitivity, selectivity, as well as thermal, chemical, and mechanical stability has pushed continued research in the SPME field. Since the fiber coating is one of the most significant factors impacting SPME function, many different approaches have been explored to improve performance. For example, the sol-gel technology physically incorporated with polydimethysiloxane (PDMS) has been shown to enhance the thermal stability and sensitivity higher than normal PDMS. The planar geometry substrate was first applied for PDMS coating in order to increase the surface area and volume of the polymer phase, which subsequently enhances the capacity of SPME.
Certain coating polymers and polymer composites (i.e. BSP3 polymer, acrylate/silicone co-polymer, polyrrole, poly(phthalazine ether sulfone ketone)) and different preparation techniques (i.e. electrochemical, molecular imprinting, physical deposition) have also been employed to SPME for improved performance of the polymer coating.
SPME with nonpolymer coatings has been explored to overcome some of weakness of polymer coatings, specifically the limited capacity, selectivity, and chemical and thermal stability. Inorganic porous sorbents with high surface area such as, carbon nanotubes, activated charcoal, and porous silica coated SPME have been explored and reported to be effective materials for improvement in the extraction of analytes for some conditions. Metals and metal oxides such as, La (III), Al, and Nb2O5 have been investigated and developed for enhanced capturing and releasing of target analytes.
Among alternative nonpolymeric SPME materials, silica and silica composites are among the most promising coating materials. Some nanoporous silica materials can have a very high surface area per unit volume, sometimes over 1000 m2/g, while retaining hydrothermal stability. Further, they are amenable to the installation of a wide range of surface chemistries. However, integrating the silica materials into a functioning SPME device is not a trivial task. Challenges include the fragile nature of fused silica support fibers (which need extra care during SPME manufacture and application), the brittleness of ceramic coatings, and the challenge of creating a uniform thin film on the silica support fiber. It is important to maintain the available surface area of nanoporous silica material during the attachment process of coating to the support. The retention of a large relative surface area offers installation of selective silane functional groups (able to obtain high density of binding sites), resulting in high sorption capacity and sensitivity to analytes of interest. Effectively attaching nanoporous material onto the SPME fibers depends upon the attachment method. A glue method utilizing epoxy has been reported for attaching functionalized/unfunctionalized mesoporous silica and other porous silica particles onto SPME supports.
Rapid detection of organic chemicals is important for a range of areas including biomedical, agricultural, industrial, environmental, forensic and a range of health and safety related areas. The threat of terrorism has heightened the significance of rapid fieldable detection of trace level organics for security concerns. SPME is an ideal approach for improving methods for sampling and analysis of the forensic signatures, chemical weapons and explosives.
What is needed is a device and method of making with improved capacity for trace analyte capture, enhanced affinity and selectivity of target analytes.